A hybrid vehicle comprises an internal combustion thermal engine, which transmits torque to the driving wheels by means of a transmission provided with a gear box, and at least one electric machine which is electrically connected to an electric storage system and mechanically connected to the driving wheels.
The following is possible while traveling: either a thermal operating mode, in which the torque is generated only by the thermal engine and the electric machine may possibly work as a generator for recharging the storage system; or an electric operating mode, in which the thermal engine is off and the torque is generated only by the electric machine working as a motor; or a combined operating mode, in which the torque is generated both by the thermal engine and by the electric machine working as a motor. Furthermore, in order to increase the overall energy efficiency during all steps of decelerating, the electric machine may be used as a generator for a regenerative deceleration in which the kinetic energy possessed by the vehicle is partially converted into electricity, which is stored in the storage system, instead of being completely dissipated in friction within the brakes.
When the electric machine is disconnectable from the driving wheels, i.e. when the electric machine is mechanically connected to a primary gear box shaft or is directly mechanically connected to a drive shaft of the thermal engine, the electric machine may be used to start the thermal engine itself, thus also serving the function of a starter motor (as described, for example, in patent U.S. Pat. No. 5,337,848A1 and in patent application GB2335404A). When the thermal engine is “cold”, the thermal engine requires the application of a very high torque (indicatively even 2-4 times higher than the starting torque needed to start a “warm” thermal engine) to the drive shaft, because due to the low temperature, the lubricant oil inside the thermal engine is not very fluid and thus opposes a high mechanical resistance. A traditional starter motor is able to apply a very high torque to the drive shaft of the thermal engine by virtue of a very high gear-down ratio (e.g. even 1:10) between the starter motor and the drive shaft; instead, the electric machine of a hybrid vehicle is normally mechanically connected to the drive shaft of the thermal engine with a low gear-down ratio (often unitary, i.e. 1:1, and thus without any gear-down). Therefore, the electric machine of a hybrid vehicle may not be capable of generating the very high starting torque needed to start the thermal engine when the thermal engine is “cold”.
In order to solve the above-described problem, in patent application US2002117860A1 it is suggested to mechanically couple the electric machine to the drive shaft of the thermal engine with two different, automatically selectable transmission ratios: a first more geared-down ratio is selected during a step of starting the thermal engine, while a second, more direct ratio (i.e. less geared-down ratio) is selected for the remaining use of the electric machine. However, this solution is constructionally complex because a servo-controlled gear box should be interposed between the electric machine and the drive shaft.
In order to solve the above-described drawback, in patent applications WO2007131838A1, US2005155803A1, U.S. Pat. No. 6,354,974A1 and US2009017988A1 the electric machine is separated from the drive shaft of the thermal engine by a clutch; in order to start the thermal engine, the clutch is opened, the electric machine is “idled” and the clutch is closed with the electric machine rotating so as to also exploit the inertia (i.e. the kinetic energy) of the electric machine for making the thermal engine rotate. However, such a “launched” starting mode may also not be sufficient to start a sporty, high-performance thermal engine which has a high displacement and with a high number of cylinders. Furthermore, the “launched” starting mode allows to apply a high starting torque to the drive shaft needed to start a “cold” thermal engine, but which is redundant to start a “warm” thermal engine; therefore, when the thermal engine is “warm”, the “launched” starting mode may unnecessarily impose high mechanical stresses on the transmission components and unnecessarily determines a certain delay when starting the thermal engine due to the time needed to “idle” the electric machine.
Patent application DE19645943A1 describes starting the thermal engine with a “launched” starting mode (which includes making the electric motor rotate before mechanically connecting the electric machine to the thermal engine) when the thermal engine is “cold” and using a conventional starting mode (in which the electric machine is mechanically connected to the thermal engine before it starts to rotate) when the thermal engine is “warm”.
Patent application GB2413998A describes starting the thermal engine with a “launched” starting mode (which includes making the electric machine rotate before mechanically connecting the electric machine to the thermal engine); the launch rotation speed (i.e. the rotation speed which should be reached by the electric machine before mechanically connecting the electric machine to the thermal engine) is determined according to the temperature of the thermal engine so that the launch rotation speed is higher when the thermal engine is “cold”.